The present invention is related to locomotive brake control apparatus and particularly to an arrangement employing step-up type relay valves.
Conventional 26-L type locomotive brake equipment employs a 26-C type, self-lapping, automatic brake valve having an automatic portion designed for regulating the train brake pipe pressure controlling both the locomotive and train brakes, and an independent portion arranged to apply and release the locomotive brakes independently of the train brakes. A double check valve connects the respective automatic and independent brake pressures to a relay valve, which in turn develops the locomotive brake cylinder pressure.
A step-up type relay valve is commonly employed by railroad properties to obtain independent brake forces greater than the automatic brake force. This is accomplished by employing two separate diaphragm control pistons. A main piston is subject to the greater of the automatic and independent brake pressures, while an auxiliary piston is subject to the independent brake pressure. Thus, when the independent brake is applied, both pistons are effective to pilot the relay valve such as to obtain the brake pressure, whereas only the main piston is effective to pilot the relay valve when the automatic brake is applied. Such an arrangement allows for more positive braking at low speeds, and provides better holding power when the train is at rest. However, these step-up relay valve arrangements have been found to cause rapid shoe wear and wheel damage due to excessive brake pressure, which can result when both the automatic and independent brakes are applied concurrently.
In order to counteract these excessive brake forces, a piloted two-position, three-way pneumatic valve has been used to limit the maximum brake cylinder pressure obtainable to a value which will avoid wheel damage and excessive shoe wear. Such arrangements typically employ a pneumatically piloted switch valve which vents the independent brake pressure from the auxiliary piston when a predetermined automatic brake pressure is produced. However, these arrangements have been found to be undesirable from a safety standpoint, in that a temporary reduction in brake cylinder pressure actually occurs when maximum effective braking is required, such as when making an emergency application on top of an independent application.